The objective of the first phase of our program was to design and build the appropriate special-purpose mass-ratio mass spectrometer based on field ionization and to determine its scope and limitations in measuring the uptake of chemotherapeutic agents and essential metabolites by normal and malignant tissues, using multilabeled molecular tracers. During the first nine months of this program we have accomplished the following tasks: The general behavior of field ionization sources and the mode of ionization compounds of interest have been examined on a 45 degrees magnetic sector mass spectrometer. The special-purpose mass-ratio instrument is at present in the last stages of construction. A number of multilabeled compounds have been synthesized especially for our program, including 4-butanediol dimethansulfonate (myleran), thymine, thymidine, uracil, uridine, 1-fluoro-uracil, and 5-fluorouridine. These compounds will be used both as tracers and as dilutants in the later phase of the program.